1. Field of the Invention
This invention relates to a control for a gas turbine engine, and particularly to an emergency or backup control for the rear compressor vanes of the engine. During normal operation the position of the vanes is controlled by a primary closed loop control system in response to selected engine operating conditions such as compressor rotor speed and fan discharge total temperature. Should a failure occur in the primary control system, the position of the vanes is locked or moves to an extreme value, and the engine may become inoperative as a result.
The present invention is directed to an emergency control which is actuated when a failure occurs in the primary control, and which provides an open loop compressor vane control in response to the engine power lever angle. The emergency control may be actuated either manually by the flight crew, or automatically in response to a sensed failure of the primary control. The actuator for positioning the variable compressor vane consists of a hydromechanical unit that will respond to both the primary control or the emergency control, thereby eliminating the need for an additional actuator for the emergency control.
2. Description of the Prior Art
Turbine engines having variable vanes in the compressor section are well known in the art. It is also known that the position of the vanes may be controlled in response to engine or ambient conditions in order to provide optimum aerodynamic operation of the engine. Advanced turbine engines utilize integrated controls which may be electronic or hydromechanical in order to control the many variable elements in the engine including the compressor vanes in a unified manner to provide high performance over the entire engine operating regime.
A major difficulty with present-day engine controls is that failure of the control may cause complete failure of the engine, and in the case of a single engine aircraft, loss of the aircraft. Completely redundant controls would add considerable expense and weight to the aircraft, and are not a practical alternative. The control of compressor vanes is critical to engine operation since there is no one vane position which will provide acceptable engine performance for all operating conditions. Consequently, a failure of the vane control which causes the vanes to become locked in a particular position or to move to an extreme operating condition may be catastrophic, and will at the minimum cause severely degraded operation over much of the engine regime.
To provide at least minimal engine operation if a failure occurs in the primary closed loop control for the variable rear compressor vanes, a simple redundant control is used as an emergency backup. Since it is not necessary during emergency situations to provide the precise control supplied by the primary control system, the emergency control may be open loop, thus eliminating the necessity for feedback from the compressor vanes. Further, the position of the compressor vanes may be scheduled in response to a single input signal such as power lever angle, thereby further reducing the complexity, cost and weight of the emergency controller. As an additional feature, a simple hydromechanical actuator may be used to position the compressor vanes in response to both the primary closed loop control and the emergency open loop control.